There have been several scanner systems developed for the purposes of detecting concealed threats and anomalies in footwear, while being worn by passengers. The systems may be based on one or more of the following three techniques: Inductive Loop, Trace Detection and Quadrupole Resonance. The three sensing techniques are based on different physical principles, but possess vulnerable components that can be easily defeated as noted below.
For example, the Inductive Loop technique uses a current loop to detect metallic objects. However, this approach faces the problem that a vast majority of footwear includes metallic parts or posts for stability, which often unnecessarily triggers alarms—a common nuisance.
The Trace Detection technique uses analysis to detect the presence of minute amounts of explosives. However, this approach faces the problem of how the lack of detection does not necessarily guarantee the absence of explosives. State sponsored terrorists, for example, would have access to a clean room where the concealment of explosives inside footwear, and would be capable of concealing explosives without any residue.
The Quadrupole Resonance technique uses material characterization via response to a known radio frequency excitation. This approach has the problem where wrapping the concealment with a sheet of conducting material (e.g., aluminum foil) would essentially shield the concealment from the RF excitation, which in effect hides the concealment from detection.